1. Field of the Invention
This invention relates to special cationic layer compounds of the hydrotalcite type, to a process for their production and to their use as stabilizers for halogen-containing plastics.
It is known that halogen-containing plastics or molding compounds produced from them tend to degrade or decompose on exposure to heat or high-energy radiation, for example ultraviolet light. To counteract this, they are normally treated with heavy metal compounds based, for example, on lead, barium and/or cadmium. For reasons of factory hygiene, however, there is a need to replace these thoroughly effective stabilizers by less health-damaging materials. Possible alternative stabilizers to the heavy metal compounds are, for example calcium and zinc soaps, but unfortunately they do not perform as well as the heavy metal compounds mentioned above so that co-stabilizers have to be used in order to enhance their stabilizing effect.
2. Discussion of Related Art
German patent DE-C-30 19 632 (Kyowa Chemical Ind.) describes the use of hydrotalcites for inhibiting the thermal or ultraviolet degradation of halogen-containing thermoplastic resins. This patent specification discloses test results which show that, when readily commercially available hydrotal-cites are incorporated, for example, in vinyl chloride resins, they accelerate the dechlorination of the resins on heating or even cause decomposition, blackening or foaming of the resins. In addition, it was found that these hydrotalcites show poor dispersibility in the resins and adversely affect the rheological properties of the resins during molding and also the appearance of the molded products obtained. These test results are attributed to the small crystal size of the usual hydrotalcites and to the large specific BET surface of at least about 50 m.sup.2 /g and the coverage of the hydrotalcite particles with water. Accordingly, it is proposed in German patent DE-C-30 19 632 to use hydrotalcites which have a large crystal size and a specific BET surface of no larger than 30 m.sup.2 /g and which may optionally be coated with an anionic surfactant, such as sodium stearate.
European patent application EP-A-189 899 (Kyowa Chemical Ind.) also describes resin compositions containing hydrotalcites having specific BET surfaces below 30 m.sup.2 /g. It is known from this European patent application that the hydrotalcites can be modified with higher fatty acid esters, anionic surfactants and coupling agents of the silane or titanium type in order to improve the compatibility of the hydrotalcite with the resins. According to cited European patent application EP-A-1 89 899, the hydrotalcites are said to be modified by mechanical mixing with the modifying agents in pure or dissolved form.
DEC-33 06 822 (Giulini Chemie) teaches that hydrotalcites with the formula [Mg.sub.6 Al.sub.2 (OH).sub.12 ](CO.sub.3).sub.2.xH.sub.2 O (with x.gtoreq.2) are obtained by reaction of aluminium hydroxide with magnesium hydroxide or magnesium oxide in the presence of basic magnesium carbonate as carbonate ion donor at a temperature of 50 to 100.degree. C. and subsequent spray drying from the suspension. The aluminium hydroxide is used in particular in the form of "active" aluminium hydroxide.
U.S. Pat. No. 4,656,156 (Aluminium Company of America) describes a process for producing hydrotalcite in which the aluminate liquor of the Bayer process is used as the aluminium component. The Bayer liquor is reacted with "active" magnesium oxide which can be obtained, for example, by calcining magnesium carbonate. This process can only be economically used in places where the Bayer liquor itself accumulates because otherwise relatively large quantities of water would again have to be transported.
DE-A-15 92 126 (Kyowa Chemical Ind.) describes the production of hydrotalcites from different starting materials, for example from a suspension of solid aluminium hydroxide, solid magnesium hydroxide and sodium hydrogen carbonate. The reactions are carried out in batches and the products are separated from the water phase by filtration or centrifugation and washed before they are dried.
According to DE-C-44 25 266 (Metaligesellschaft AG), the familiar subject of stabilizing halogen-containing plastics is outlined as follows: halogen-containing thermoplastic resins, such as polyvinyl chloride, are converted during processing (melt forming) into polyene structures with elimination of hydrogen chloride (HCl). The polymer becomes discolored. In order to improve thermal stability, metal carboxylates ("metal soaps") are normally incorporated in the resin as stabilizers. However, since the incorporation of such substances as sole stabilizers can lead to so-called metal burning in the event of relatively prolonged melt forming processes, resulting in blackening of the polymer, it is standard practice to add a co-stabilizer. Typical co-stabilizers are, for example, polyols, organic phosphorous acid esters or epoxy compounds. According to the teaching of DE-C-44 25 266, special lithium-ontaining layer lattice compounds can be used for stabilizing PVC in particular. In addition, DE-A-44 25 275 (Metaligesell-schaft AG) also teaches the use of similar Li-containing layer lattice compounds for stabilizing halogen-containing plastics.